use absolute paths on darwin and windows (#191)

We want to make sure to use the system binaries, and not whatever is in
the PATH.

tun_darwin.go

@@ -42,18 +42,18 @@ func (c *Tun) Activate() error {
 	c.Device = c.Interface.Name()
 
 	// TODO use syscalls instead of exec.Command
-	if err = exec.Command("ifconfig", c.Device, c.Cidr.String(), c.Cidr.IP.String()).Run(); err != nil {
+	if err = exec.Command("/sbin/ifconfig", c.Device, c.Cidr.String(), c.Cidr.IP.String()).Run(); err != nil {
 		return fmt.Errorf("failed to run 'ifconfig': %s", err)
 	}
-	if err = exec.Command("route", "-n", "add", "-net", c.Cidr.String(), "-interface", c.Device).Run(); err != nil {
+	if err = exec.Command("/sbin/route", "-n", "add", "-net", c.Cidr.String(), "-interface", c.Device).Run(); err != nil {
 		return fmt.Errorf("failed to run 'route add': %s", err)
 	}
-	if err = exec.Command("ifconfig", c.Device, "mtu", strconv.Itoa(c.MTU)).Run(); err != nil {
+	if err = exec.Command("/sbin/ifconfig", c.Device, "mtu", strconv.Itoa(c.MTU)).Run(); err != nil {
 		return fmt.Errorf("failed to run 'ifconfig': %s", err)
 	}
 	// Unsafe path routes
 	for _, r := range c.UnsafeRoutes {
-		if err = exec.Command("route", "-n", "add", "-net", r.route.String(), "-interface", c.Device).Run(); err != nil {
+		if err = exec.Command("/sbin/route", "-n", "add", "-net", r.route.String(), "-interface", c.Device).Run(); err != nil {
 			return fmt.Errorf("failed to run 'route add' for unsafe_route %s: %s", r.route.String(), err)
 		}
 	}

tun_windows.go

@@ -47,7 +47,7 @@ func (c *Tun) Activate() error {
 
 	// TODO use syscalls instead of exec.Command
 	err = exec.Command(
-		"netsh", "interface", "ipv4", "set", "address",
+		`C:\Windows\System32\netsh.exe`, "interface", "ipv4", "set", "address",
 		fmt.Sprintf("name=%s", c.Device),
 		"source=static",
 		fmt.Sprintf("addr=%s", c.Cidr.IP),
@@ -58,7 +58,7 @@ func (c *Tun) Activate() error {
 		return fmt.Errorf("failed to run 'netsh' to set address: %s", err)
 	}
 	err = exec.Command(
-		"netsh", "interface", "ipv4", "set", "interface",
+		`C:\Windows\System32\netsh.exe`, "interface", "ipv4", "set", "interface",
 		c.Device,
 		fmt.Sprintf("mtu=%d", c.MTU),
 	).Run()